1. Field of the Invention
The present invention relates to an image processing apparatus and a method for controlling the same.
2. Description of the Related Art
An image forming apparatus executes an exposure process by irradiating a charged photosensitive member with a laser beam. The image forming apparatus develops an electrostatic latent image generated by the exposure using toner, and transfers the developed image to a sheet to fix it.
In an exposure unit of such an image forming apparatus, a laser scanning line is ideally a straight line parallel to an axis of the photosensitive member. However, an error in attaching position or tilting of a light source or the photosensitive member may cause the laser scanning line to be distorted as compared with a straight line parallel to the axis of the photosensitive member.
Japanese Patent Application Laid-Open No. 2004-170755 discusses a method for correcting distortion of a laser scanning line. The method discussed in Japanese Patent Application Laid-Open No. 170755 measures distortion of the laser scanning line by using a sensor, corrects bitmap image data to cancel the distortion, and performs image formation based on the corrected image data.
Referring to FIGS. 18 to 20, the method for correcting the image data will be described.
FIG. 18 illustrates a data structure of image data before it is subjected to correction processing according to a conventional art. In FIG. 18, the image data is bitmap data formed into a matrix structure.
FIG. 19 illustrates a status where the image data is subjected to correction processing at a specific line according to the conventional art. The distorted laser scanning line (dark line on FIG. 19) is distorted by as much as two pixels on each side of the ideal (straight, parallel) scanning line. The image data is corrected (by an amount shown by the arrows in FIG. 19, which represent a correction value for each scanned pixel) for each line during printing as illustrated in FIG. 19, and then output to an exposure unit. As illustrated in FIG. 19, at each line, for each pixel, processing for replacing a pixel present on an ideal scanning line with a pixel present on an actual scanning line is executed.
FIG. 20 illustrates a data structure of image data after it has been subjected to correction processing according to the conventional art. In FIG. 20, the image data is distorted symmetrically to an actual scanning line in a sub-scanning direction. Use of this image data cancels distortion at the actual scanning line for printing, enabling acquisition of an electrostatic image similar to that exposed by an ideal scanning line by the exposure unit.
The ideal scanning line is a scanning line indicating, in a state where mounting positions of the exposure unit and the photosensitive member to the apparatus are ideally fixed, an irradiated position on the photosensitive member when the exposure unit irradiates one line of the photosensitive member with the laser beam.
The actual scanning line is a scanning line indicating, in a state where the exposure unit and the photosensitive member are actually mounted to the apparatus, an irradiated position on the photosensitive member when the exposure unit irradiates one line of the photosensitive member with the laser beam.
According to the method discussed in Japanese Patent Application Laid-Open No. 2004-170755, however, the image data is corrected in the sub-scanning direction for a pixel of each line. Thus, implementing the method discussed therein needs line buffers of a number of lines larger than a distortion width of a laser scanning line.
For example, when a distortion width of a laser scanning line is equal to N lines, line buffers that can store image data of the N lines are necessary. Securing many line buffers increases a memory capacity and a circuit size for correcting the image data, resulting in higher costs.